Let me help you nimrods - this is a choice between the moral and the pragmatic. The moral position being that slavery is always wrong and can never be justified. The pragmatic being that if a race is greatly elevated over time as a result of slavery, then that slavery must be viewed as positive. Conservatives never have a problem with this, they say slavery was wrong. That puts liberals in the position of saying that the conservatives are saying slavery was wrong only because it means the black race never happens in America, and they, being conservatives, are racists. But it also puts liberals in the position of saying that without slavery we would have no blacks, no Obama, no fundamental justification for liberalism, therefore no - they wouldn't go back and eradicate slavery if they could.
What this very old question really does is force the respondent to closely examine his beliefs (or cowardly not respond). Conservatives take a simple moral view and they don't have to think about it. Liberals have to agonize with "oh no if I say that it means.... b-but if I say that it sounds like..... oh my! oh my!" And that's the difference between conservatives and liberals - genuine core values.